This invention relates generally to ion implantation systems used in the fabrication of semiconductor devices and, more particularly, to systems for the control of charging currents absorbed by semiconductor wafers subjected to the implantation processing. Ion implantation is an important process step in the fabrication of integrated circuits on semiconductor wafers. Although the high density and speed of modern integrated circuits are largely a result of improvements in the accuracy and resolution of lithography and etching apparatus used in the manufacturing process, the density and speed are also dependent to some degree upon tight control of the profile of doped regions of the wafer. Control of doping of wafer regions with conductivity-modifying impurities can best be achieved using ion implantation techniques.
In a typical ion implantation process, semiconductor wafers are subjected to a broad and fairly uniform beam of ions, or charged atoms. A region of the semiconductor wafer can have its conductivity properties modified by subjecting it to bombardment by a beam of ions of a selected impurity. Regions to be treated by ion implantation are defined on the semiconductor wafer by an etched pattern in a previously formed layer known as a resist layer. The depth of implantation and the resultant properties of the region are largely dependent on the beam properties, such as the energy of the ions, beam density, time of exposure to the beam, and so forth. These considerations are now well understood in the art of semiconductor fabrication, and are beyond the scope of the present invention.
The present invention is concerned with only one aspect of the ion implantation process, namely a difficulty that arises from the buildup of electrical charge on the wafer, due to its bombardment by charged particles. For example, when a semiconductor gate region is subjected to ion implantation treatment, it becomes positively charged with respect to an underlying substrate, from which it is typically separated by a dielectric layer. Without intervention, the gate may become so highly charged that the dielectric material breaks down, or becomes structurally weakened. One well known solution to this difficulty is to use an electron flood device to add electrons to the beam. Because the electron and ion velocities are substantially different and the electrons are spatially separated from the ions, there is little, if any, reaction between the ions and electrons in the beam, and the electrons have no significant effect on the ion implantation process. However, if the generation of electrons is properly controlled the net charging current applied to the semiconductor wafer can be reduced to practically zero.
Up to this point, it has been tacitly assumed that only one semiconductor wafer is being treated, and that the ion beam is large enough in cross section to treat the entire wafer. As a practical matter, this is not the case. A typical semiconductor wafer may be about 6 inches (15 cm) in diameter, but it is impractical, or at least very expensive, to produce an ion beam of more than about 5 cm in cross-sectional width. Accordingly, there must be some technique for scanning the beam across the wafer, or scanning the wafer with respect to the beam. Further, it is desirable from an efficiency standpoint to be able to treat many wafers at the same time in a single apparatus.
A typical processing device for ion implantation can treat twenty or more wafers together by having them mounted on the spokes of a large wheel, which is rotated at high speed (about 1,250 rpm) through the ion beam. This exposes successive wafers to the beam, but does not expose the entire width of each wafer. Therefore, the wheel is also angularly precessed. That is to say, its axis is scanned laterally from side to side at a relatively slow rate, to ensure that the entire width of each wafer is exposed to the beam.
In ion implantation systems of the prior art, the net charging current to which the wafer-supporting wheel is subjected is measured, and used to control operation of the electron flood device, in an effort to neutralize the charging current. The wheel is electrically conductive, and the wafers are carried on heat sinks on the wheel spokes, to help dissipate the heat generated as a result of the ion implantation process. Electrical contact with the wheel is made through a slip-ring or similar device at the wheel hub; and the magnitude of the sensed current is used in a conventional control system to vary the rate at which electrons are injected into the beam.
Although the typical prior art control system described above operates satisfactorily in most respects, it has some significant drawbacks, due in part to variations in the charging current as the wheel turns about its axis. One such variation is caused by the potential absence of wafers from some positions of the wheel. If less than a full batch of wafers is being processed, the empty wafer positions are usually filled by dummy wafers, to avoid damage to the wafer heat sinks if they were exposed to the ion beam. However, the dummy wafers do not have exactly the same physical and electrical characteristics as the real wafers. One component of charging current results from secondary electrons dislodged from the wafers by impinging ions. This secondary effect may be significantly different on the dummy wafers, so that a current measured as the "average" of charging currents on all of the wafers may not be an appropriate control signal to neutralize charge on the real wafers. Another variation in charging current occurs as a result of the beam's impingement on portions of the wheel other than where a wafer is carried. Because the wafers have to be scanned laterally across the beam, providing this full beam coverage of the wafers requires that the beam must also impinge on other portions of the wheel. Secondary emission effects when the beam is impinging on the wheel itself are also different from corresponding effects when the beam is positioned over a wafer. Therefore, inclusion of these effects in the measured "average" charging current yields a control signal that is not an accurate measure of the charging current applied to the wafers.
Accordingly, there is still a need for improvement in the control of ion implantation beams, to neutralize charging current applied to the wafers being processed. The present invention is directed to this end.